[Az.Blockchain] Retirement message

[dcaro]

Description

Following the announcement of the Azure Blockchain service retirement we plan to update Az.BlockChain module:

[ ] Update Az.BlockChain module release notes
[ ] Update the modules PSD1
[ ] Update list of Az PowerShell modules - https://github.com/Azure/azure-powershell/blob/master/documentation/azure-powershell-modules.md

Cost

[dingmeng-xue]

@dcaro, I prefer to the first 2 items in upcoming release. Regarding documentation, https://github.com/Azure/azure-powershell/blob/master/documentation/azure-powershell-modules.md is just one. We need to remove their cmdlet reference together. We need to update azure-powershell-modules.md when we delete this module from repo.

[dingmeng-xue]

Following items will be tracked by #15233